(a) Field of the Invention
This invention relates to a retrofocus type superwide angle lens system having a long backfocus.
(b) Description of the Prior Art
In such retrofocus type superwide angle lens system as of a superwide angle in which the field angle of view exceeds 90 degrees and a long backfocus, an image may collapse in the peripheral portion of the field of view mostly because the chromatic aberration of the magnification is large near the maximum field angle of view. Such collapse of the image because of the chromatic aberration of the magnification is different from the collapse of the image because of the coma and can not be eliminated even by a smaller stop aperture. Therefore, the deterioration of the image caused by the chromatic aberration of the magnification can not be eliminated by adjusting the stop aperture or the like and always occurs. For such reason, the chromatic aberration of the magnification must be made as small as possible. In the case of a retrofocus type superwide angle lens system, the chromatic aberration of the magnification varies extremely large with the change of the field angle of view. Therefore, usually the chromatic aberration of the magnification is undercorrected at the intermediate field angle of view but is overcorrected at the maximum field angle of view so as to keep the balance of the amounts of the chromatic aberrations of the magnifications at the respective field angles of views. However, when the field angle of view reaches even 100 degrees, if the correction is made to well balance them at the intermediate field angle of view, it will be remarkably in excess at the maximum field angle of view. In order to eliminate this defect, the difference of the chromatic aberration of the magnification by the change of the field angle of view must be made small.
Further, in such superwide angle lens system in which the field angle of view reaches 100 degrees, in case a filter is used as fitted in front of the lens system, in order that the light of the maximum field angle of view may not be cut, the filter diameter must be made so large as to be undesirable.